1. Field of the Invention
This invention relates to a process for surface-treating copper products, for example, a copper foil used for printed circuits or the like. More particularly it relates to a process for surface-treating a copper foil by making ternary alloy of zinc-nickel-copper on the surface of the foil, which prevents reduction of peel strength between copper foil and base material caused by corrosion with an etching solution or an aqueous solution of hydrochloric acid, etc.
2. Related Art Statement
Copper-clad laminates have usually been used for printed circuits and prepared by bonding a copper foil a base material. The surface of the copper foil with which the base material is bonded has been subjected to various chemical or electrochemical treatments. Namely, in order to enhance the bond strength of the foil onto the base material, a granular copper layer has been electrically deposited on the surface of the copper foil to form a roughened surface, followed by coating the roughened surface of the electrically deposited copper with a metal such as zinc, tin, nickel or brass (U.S. Pat. No. 3,585,010, U.S. Pat. No. 3,377,259, U.S. Pat. No. 2,802,897).
This zinc layer prevents the granular copper from migration, and also prevents spotting or staining after etching the copper foil. The layer has a thickness to such an extent at the high bond strength of the granular copper layer is not reduced.
During lamination said copper foil is usually heat pressed. By application of heat and pressure, zinc layer is converted into brass by mutual diffusion between the zinc layer and copper substrate and exhibits yellow color. Thus after laminating the copper foil with base material, the resulting boundary forms a brass layer.
When a printed circuit is produced using said copper-clad laminate, the following processes are employed: The copper-clad laminate is punched or drilled. The inside of the holes are activated, and copper is uniformly deposited thereon by means of non-electrolytic plating, followed by electrolytic plating to increase the copper thickness. Then photoresist is applied onto said copper foil and a pattern etching is carried out to obtain a desired printed circuit.
During the activation treatment of the inside of holes and during the pattern etching, the boundary layer i.e. the brass layer is exposed to a solution containing hydrochloric acid. Further, after the pattern etching, various plating processes may often be carried out. In such case, the brass layer is exposed several times to various plating solutions and acidic solutions. Since the brass layer is hardly resistant to hydrochloric acid the brass layer is corroded due to dezincification so that peel strength between the copper foil and the base material is likely to be reduced.